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Unusually high alfalfa weevil populations

May 03, 2010

Alfalfa weevil populations appear to be unusually high this spring and treatments do not seem as effective as in years past, a Kansas State University entomologist said.

"Alfalfa weevil infestations continue to cause concern in southwest, south central and north central Kansas," said Jeff Whitworth, entomology specialist with K-State Research and Extension. "This is the largest population of weevils that I´ve seen in at least 10 to 12 years in Kansas."

In a typical year, it would be common to find one larvae per two to three stems, he said. This year, however, two to three larvae per stem are common, especially south of Interstate 70.

Whitworth said that inconsistent insecticide results have sparked talk among some producers of insecticide resistance. He believes, however, the large number of larvae is probably due to a combination of a massive infestation that continued to hatch over a three- to four-week time frame and variable weather including hot, dry periods and cool, wet periods that likely contributed to the inconsistent insecticidal efficacy.

"Most products provided 60 to 75 percent control, but with infestation levels like we have this year, this may not seem like acceptable performance," he said. "But there is no comparison to untreated fields that have little left to harvest."

Whitworth said that larvae in south and central Kansas will likely start to pupate in the next seven days or so, thus feeding and consequent damage will not continue at the same level as in the past three weeks.

"If you have already treated once or twice but are still not satisfied with the control, if you´re within 10 days of harvesting, it may be best just to cut a little early instead of re-treating," he said. "If you do treat with an insecticide, always pay attention to the pre-harvest interval (PHI) on the label. If you did treat but there are still a considerable number of larvae feeding in the field, you may want to pay close attention under the windrows after it is swathed. There may be more adults in the fields than in past years and they will feed in new growth until it gets warm enough to drive them from the alfalfa to their over-summering sites."